The present invention relates to a transmission line switching system comprising a plurality of normal transmission lines and standby transmission lines, in which when a failure occurs on any of the normal transmission lines, the normal transmission line is switched to one of the standby transmission lines, and when the switched normal transmission line is recovered, control returns to the recovered normal transmission line.
In such a transmission line switching system, for example see that disclosed in the publication of "Computer-Aided Digital Transmission Protection Switching System" by Yasuyuki Suzuki, JTR January, 1983, PP. 39 to 48, switching operations must be simultaneously performed on a single transmission line at both transmission and reception terminals. In order to accurately perform switching at the transmission and reception terminals, a transmission line used exclusively for the transmission of switches, information is connected between both the terminals.
In a conventional system, an extra installation cost is therefore required for an exclusive transmission line, resulting in an economical disadvantage. In addition, upon switching to a standby transmission line and subsequent returning to a normal transmission line which has been recovered, no normality check is performed for either the switching destination or switching returning destination. Therefore, if a failure also occurs on a standby transmission line or, if a normal transmission line which is determined to be recovered is still incomplete, communication is undesirably interrupted.